Method of aluminum coating a ferrous base with a molten solution of aluminum in magnesium



United States Patent METHOD OF ALUMINUM COATING A FERROUS BASE WITH A MOLTEN SOLUTEON 0F ALUMI- NUM IN MAGNESIUM Andrew Dravnieks, Park Forest, llllt, assignor to Standard Oil Company, Chicago, lll., a corporation of lndiana No Drawing. Filed Sept. 25, 1961, Ser. No. 140,259

5 Claims. (Cl. 117-114) This invention relates to the impregnation of a base metal with a second metal capable of forming a solid alloy therewith. More particularly, this invention relates to a method of aluminizing ferrous base metal wherein the base is impregnated with a controlled amount of alummum.

There are a large number of commercial applications for structures comprised of a base metal impregnated or coated with a second metal wherein the second metal is employed to protect the base metal or to otherwise impant improved properties to the structure. Typically, aluminizing is widely used to protect a base metal, such as carbon steel or other ferrous alloys, against oxidation and sulfidization at hightemperatures. There are a number of well-known processes for impregnating steels with aluminum. Such processes typically involve alloying the surface of the ferrous base metal with aluminum by diffusion methods. In such processes the base metal may be packed in a powder compound containing aluminum, dipped in molten aluminum or sprayed with an aluminum coating. Subsequent to the placement of the aluminum on the surface of the base metal it is the practice to heat the coated base metal for a long period of time at temperatures in the range of 1500 F. to 1800 F. to cause the aluminum to penetrate the base metal by diffusion. The result is a surface layer of iron-aluminum alloy, usually ranging from 0.005 to 0.040 inch in thickness and containing a relatively high concentration of aluminum, e.g., approximately 25 weight percent or more aluminum.

In the iron-aluminum system there are several intermetallic compounds formed at definite ratios of aluminum in iron. For instance, as shown in the aluminum-iron equilibrium phase diagram published in the Metals Handbook, 1948 edition, published by the American Society for Metals, Cleveland, Ohio, page 1161, the intermetallic compound formed in this binary system which contains the lowest amount of aluminum and which is present at room temperature occurs at approximately 12 weight percent aluminum (Fe 'Al). At approximately 33 Weight percent aluminum another intermetallic compound, FeAl, is formed; while additional intermetalli'c compounds are formed in higher aluminum ranges, e.g., about 50 to 60 weight percent aluminum. Such intermetallic compounds are readily formed as a result of conventional aluminizing methods because of the uncontrolled amounts of aluminum which are present for difiiusion into the base metal. characteristically, such intermetallic compounds are relatively hard and brittle, and when they occur in aluminized coatings there is a tendency for the coating on the base metal to crack, spall, and allow a corrosive to more readily attack the base metal.

The general object of the present invention is a method of impregnating a solid base metal with a second metal to provide a structure having improved properties. A further object of the invention is a method for impregnating a base metal with a second metal whereby alloys of controlled composition are formed, and whereby the formation of undesirable intermetallic compounds can be avoided under controlled conditions. Still a further object is such a method whereby coatings of a controlled thickness are readily applied to la. base metal which coatings have an excellent bond to the base metal and are free from inclusions, gas pockets and other similar defects.

An additional object of the invention is such a method for aluminizing a ferrous base metal wherein additional heat treatment is not required following the coating step.

It has now been found that a solid base metal can be impregnated with an element capable of forming a solid alloy therewith by contacting the base metal with a liquid solution containing a preselected amount of the element in a solvent metal at an elevated temperature which provides a desired rate of diffusion of the impregnating element into the base metal. In the process of the invention a solvent metal is selected which is capable of forming a liquid solution with the desired amount of the impregnat ing element, but which solvent metal is immiscible with the base metal under the conditions of the process. The ratio of the impregnating element to the solvent metal is controlled to provide a predetermined chemical potential of the impregnating element in the liquid solution which produces, under the conditions of the process a desired alloy of the impregnating element with the base metal without the formation of undesired alloy compositions.

To illustrate, if a liquid solution of a solvent metal, A, and a solute metal, B, is contacted with a base metal, C, with which metal B will diffuse into to form an alloy and in which metal A is essentially insoluble under the conditions of the process, there is a tendency for the solute metal atoms to be transferred from the liquid system to the solid system, or from the solid system to the liquid system, depending upon the differential in the chemical potential of the solute metal in each of the systems. The chemical potential is the partial molar free energy, i.e., the free energy required to remove one mole of a substance (solute) from a particular phase (liquid system or solid system). The differential chemical potential, then, is a measure of the tendency for the solute atoms to be transferred between the liquid phase and the solid phase. Of course, the chemical potential of a particular phase will vary with the composition of the phase. Thus, the composition of a solid alloy formed on a base metal in contact with a liquid phase containing an element which will diffuse into the base metal at an elevated temperature can be varied precisely by controlling the composition of the liquid phase.

According to the present invention there is provided a method of impregnating a solid base metal with a controlled amount of a second metal capable of forming a solid alloy with the base metal which method comprises contacting the base metal with a liquid solution of the second metal in a solvent at an elevated temperature at which the second metal is capable of difiusion into the base metal, the solvent metal being essentially insoluble in the base metal under the conditions of the process, and wherein the ratio of the second metal to the solvent metal in the liquid solution is controlled to provide a predetermined differential in the chemical potential of the second metal in the liquid solution and that of the second metal in a particular solid alloy of the second metal-base metal system.

In a preferred embodiment of the present invention a solid ferrous base metal is impregnated with a controlled amount of aluminum by a method which comprises contacting the base metal with a liquid solution of aluminum in magnesium at a temperature above about 850 F. and below the solidus of the aluminum-iron system, wherein the ratio of aluminum to magnesium in the liquid solution is controlled to provide a chemical potential of the aluminum in the liquid solution which produces an aluminum-iron alloy layer on the base metal which is free of aluminum-iron intermetallic compounds. Advantageously, the concentration of the aluminum in the liquid solution is between about 0.1 and 20 atomic percent, and preferably, the concentration is maintained between 0.5

7 1 'jent invention. I

The thickness of the surface 'alloy layer will be deter mined by exposure timeand temperature principally. The composition of the alloy formed by diffusion will be de- 'termined by the concentration of theimpregnating eleand ltl'atomic percent. "Advantageously, the aboveproc- 'e'ss iscarried'iou-t under an inert atmosphere, such as i helium, which is notreactive with any of the components of either systemunder the. operating conditions.

i The method of the 'presentinventionmaybe employed I in impregnating or coating. a number. of diiferennbase jimetals with fa variety of other elements, Base metals which contain. significant amounts of the transitional metals, such asiron," manganese, chromium, molybdenum 3 or nickelmayibe employed Typically, carbon steels and low alloy steels are well suited for aluminizing. v Other. elements which will diifuseintothe base metal to ailoy with thebase metal and which impart the desired prop vant ag 'eous for the particular articleto be impregnated may be utilized. For instance, fabricated process heaters, heat exchangers piping systems and vessels may be aluminized, either before or after assembly, by pumping the aluminum-magnesium melt through such units.

niques for pumping such liquid metals are-well known and n Tech- .need not be described herein V erties to the base metal maybe used to impregnate the f "base metal. Advantageously, aluminum is utilized to im- "pregnate thegbas e metal, however other elements such as copper, silicon, etcQmay'b'e used. The solventemployed should' be essentiallyinsoluble in the base metal, but, :s hou'ldform a liquid 'solution with the aluminum or other 'impr'egnating element within a broad range ,which ,en-

compasses the desired amounts of the impregnating ele- ."mentg. The solvent metal may be capable ofhdissoiving' a smallamount 'ofthebase metal which does not interfere "with the impregnation process. 3 Magnesium is. well suited for theabove-mentioned systems. Phase equilibria Z'diagrams for-a large number of systems are 'well known .and readily availableto enable one to select the various components which may be used in the practice er the presmentin the liquid solution. Forl'examplej with a given 'Al' Mg solution the composition of the Al-lFe diffusion layer remains the. same even with long periods of exposure. I v The temperature employed in thepresent'process is at least as high as'the lowest melting compositions formed between the impregnating metal and the solvent metal and I The followingexamples are given to illustrate the practi'ce of a preferred embodiment of the present invention. 7 :However, the examplesare givenfor illustrative purposes only, and do not serve as alimitation on the present J invention. i i v Example I 1 1 j a Test coupons of ASTM A-zss' carbon s reerwere re pared and immersed in a molten solution of magnesium, andlaluminum underia heliumatmosphere at atmospheric pressure. The liquid magnesium-aluminum solution contained'S atomic.percentaluminum. One of the test couons was contacted with the liquid solution at 1750 .E.

for one hour, after which thetestcoupon was removed' from the. bath, cooled and examined metallographically.

'A thin diifusion layer .of' aluminum-iron alloy' was observed on the, base metal.

' Other test coupons were contacted with the above liquid solution at 1750 F. for 4' hours and for 16 hours. After being removed from the bath these latter'coupons were. "examined and found to contain somewhat thicker diffusion layers of, aluminum-iron alloy, the thickness of the coupon exposed for 16 hours being approximatelylsmils.

Another couponwas contactedwith theabove-men- 'tioned liquid solution at .1450 F. for 4 hours.- The re-.

suiting 'difiusion layerwas somewhat thinner than that 5 obtained at the highertemperature, but was readily; ap-

4n should be high enough to permit a practical high rate of diffusion of the impregnating element into the base .rn'etal; Furthenfa temperature should be employed at which the rate of attack; by the solventmetal 'on the base metal is -"not harmful. The maximum temperature, of course, will 'be determined by the melting point of the alloy formed between the base metal and the impregnating metal, and

' .should be selected accordingly.- Typically, when impregnating a ferrous base metal, such as carbon or a low alloy 7 steel, with aluminum from a magnesium. liquid solution the temperature may range from about 850 F. to about 2150" F. and,preferably is at least above 1200 F., e.g., between about .1500 F. and 1800F. The pressure at whichfthe process is carried out may vary over a, wide range from sub-atmospheric to super-atmospheric pres sures' which prove to be most advantageous. .In general, howeven'fthe effect of varying pressure on the rate of diffusionis small comparedto the effects of temperature,

solvent concentration and time.

, 'Advantageously, the. surface of the-base metal is thoroughly cleaned of scale, such as oxides and sulfides, by" I mechanicalor chemicalme'ans prior to contacting'it with I the liquid solution containing theimpregnatingelement."

ln general; higher temperatures and longer times in- A crease the thickness of :the'surface "alloy layer formed on .theIbase metal. For instance when aluminizing steel then .thickn'essoi the aluminum-iron alloy'layer varies. roughly I proportionally. tothe square root of time. Thu s, higher ."lcontacting' temperatu'res may be employed to lessen the contacting .t'ime'frcquired to producean alloylayer of a,"

desired thickness. This diffusion mechanism is well known .in the art and described in detail in publications 1 i i jisuch asthe Metals Handbook hereinabove referred to.

The presentfprocess maybe employed,, typically, by in immersinga'ferrous base metal article ihtothe liquid;aluminum-magnesium solution, or other techniques most adparent uponmetallographic examination.

The diffusion layers from the above test coupons were analyzed by X-ray diffraction. parameters obtained, the'difiusion layers. containedlan estimated l0.to 20 percent aluminum. Nointermetallic compounds were found to' be present, all of the diffusion layerbeing a solution of aluminum in ferrite. Y

i Example 11 Test coupons ofthe type described above were prepared 7 and immersed in a molten magnesium-aluminum solution containing 3 atomic percent aluminum at 1750 F. for

varying lengths of time, i.e.,1 hour, 4 hours, and 16 hours.

'No intermetallic compounds were observedi in the dif fusionlayer after the above treatment. The thickness. of

the diffusionlayer of the coupon exposed forone hour was observed to be in the r'ange'of 3.04 to 3.2"mils. 'The thicknessofthe coupon exposed for 4 hours was observed to be in the range of 4.1 to 4.3 mils. The thicknessof the diffusion layer on the coupon exposed for 16 hours was observed to beinthe range of 12 to-16 mils. No inclumetal w th acontrolledamount of aluminum which method comprises contactinga surface ofsaid' base metal, 7 free of scale, with a liquid solution of aluminum in mag-' *sions' or gas pockets were observed in the alumim'zed coupons. P v f From the aboveexamples itis apparent that the method --of the present invention can be employed to'impregnate i a'base metal withanothermetal'to provide alloys or surface alloy layers of a controlled composition'. From the i foregoing. descriptionv of the invention various modifications in the materials and methods of operation will become apparent to the skilled artisan, and, as' such, fall within the spirit and scope of thepresent invention WhatIclaimisz i 1. The methodof impregnating a solid ferrous base i nesium at a temperature between about 850 F. and about 1 minurn.

2150 F. for at least about on'e'hour, said liquid solution containing between about 0.1 and 20 atomic percent aluminum.

2. Themethod of claim -1 wherein solution contains betweenabout 0.5 and 10 atomicpercent alu- Based upon the lattice 3. The method of claim 1 wherein said liquid solution contains between about 0.5 and 5 atomic percent aluminum.

4. The method of claim 1 wherein said contacting is carried out in a helium atmosphere.

5. The method of aluminizing a steel article which comprises contacting a surface of said article, free of scale, with a magnesium-aluminum liquid solution at a temperature between about 1200 F. and 1800 F. under an inert helium atmosphere for at least about one hour to provide an aluminum-iron diffusion layer of a desired thickness on the surface of said article, said solution containing about 0.5 to 10 atomic percent aluminum.

References Cited in the file of this patent UNITED STATES PATENTS Moe et al May 1, 1951 Kanter Nov. 10, 1959 Linden May 3, 1960 Sprowl Sept. 25, 1962' FOREIGN PATENTS Germany Mar. 27, 1941 

1. THE METHOD OF IMPREGNATING A SOLID FERROUS BASE METAL WITH A CONTROLLED AMOUNT OF ALUMINUM WHICH METHOD COMPRISES CONTACTING A SURFACE OF SAID BASE METAL, FREE OF SCALE, WITH A LIQUID SOLUTION OF ALUMINUM IN MAGNESIUM AT A TEMPERATURE BETWEEN ABUT 850*F. AND ABOUT 2150*F. FOR AT LEAST ABOUT ONE HOUR, SAID LIQUID SOLUTION CONTAINING BETWEEN ABOUT 0.1 AND 20 ATOMIC PERCENT ALUMINUM. 